The present invention relates to a system for detecting an abnormality of an automotive engine for a motor vehicle, and more particularly to a system which detects an abnormality by deviations of learning coefficients.
In one type of electronic fuel-injection control, the amount of fuel to be injected into the engine is determined in accordance with engine operating variables such as mass air flow, engine speed and engine load. The amount of fuel is determined by an injection pulse width. Basic injection pulse width T.sub.P can be obtained by the following formula. EQU T.sub.P =K.times.Q/N
where Q is mass air flow, N is engine speed, and K is a constant.
Desired injection pulse width T.sub.i is obtained by correcting the basic injection pulse T.sub.P with engine operating variables. In a learning control system, the desired injection pulse width is calculated by a following equation. EQU T.sub.i =T.sub.P .times.(COEF).times..alpha..times.K.sub.a
where COEF is a coefficient obtained by adding various correction or compensation coefficients such as coefficients on coolant temperature, full throttle open, engine load, etc., .alpha. is a feedback correcting coefficient of an O.sub.2 -sensor provided in an exhaust passage, and K.sub.a is a correcting coefficient by learning (hereinafter called learning coefficient). Coefficients, such as coolant temperature coefficient and engine load, are obtained by looking up tables in accordance with sensed informations. The value of the learning coefficient K.sub.a is derived from a RAM in accordance with engine load.
In order to obtain these informations, various sensors are provided in the engine. Those sensors inherently deteriorate in output characteristics with time. Accordingly, if the air-fuel ratio deviates largely from a desired air-fuel ratio because of the deterioration of a sensor, a warning for abnormality of the engine should be given to a driver of the vehicle.
Japanese Patent Laid Open No. 55-112695 discloses a diagnose system in which the number of occurrences of an abnormal signal from a sensor is counted, and when the number exceeds a predetermined number, a warning is given.
However, in the engine, since the output of a sensor varies largely in accordance with engine operating conditions, such a system is not available.
On the other hand, in the learning control system, all the learning coefficients are arranged in a form of a lookup table comprising a plurality of rows and columns in accordance with the engine load. Coefficients in divisions at intersections of rows and columns are initially set to the same value, that is the number "1". This is caused by the fact that the fuel supply system is to be designed to provide the most proper amount of fuel without the coefficient K.sub.a. However, every automobile can not be manufactured to have a desired function, resulting in same results. Accordingly, the coefficients K.sub.a are updated by learning at every automobile, when it is actually used. If an abnormality occurs in the engine, the learning coefficients are largely changed by the updating. When a coefficient in a division exceeds a predetermined limit range, the division is registered as an abnormal division When the number of registered abnormal divisions exceeds a predetermined number, it is determined that the air-fuel ratio control system becomes abnormal. The abnormality is warned and the value of each coefficient is set to one for the fail-safe.
There are a common driving condition range in which the motor vehicle is commonly driven and common divisions included in common driving condition range. Accordingly, the common divisions are frequently updated and liable to be registered as abnormal divisions earlier than other divisions. If the predetermined number of the abnormal divisions for the detection of abnormality is larger than the number of the common divisions, the coefficients in divisions other than the common divisions are rarely updated. As a result, the detection of abnormality retards. To the contrary, if the number of the abnormal divisions is smaller than the number of the common divisions, the system is regarded as abnormal in spite of slight noises.